1. Field of the Invention
The present invention relates to an image retrieval method, and more particularly, to an image retrieval method and apparatus independent of an illumination change.
2. Description of the Related Art
There are a variety of image retrieval methods. One of them is a text-based image retrieval method in which text is attached to an image and by accessing the text, a desired image is retrieved. When an image is retrieved using text, there are limitations in the text expressions used to describe a picture. Also, semantic interpretations of an image vary for different users who retrieve the image such that cases frequently occur where an image totally different from an image which a user desires to retrieve is retrieved and output.
Another method is a content-based image retrieval method in which the major characteristic features of an image desired to be retrieved are used in retrieving the image. Major characteristic features in an image used in the content-based image retrieval method include a color, texture, shape, motion information, etc. When an image is retrieved using these major characteristic features in the image, the probability that an image desired to be retrieved by a user is found increases. Accordingly, this method can reduce retrieval errors that occur due to the semantic disagreement in text-based image retrieval.
In color-based retrieval, which is a type of content-based retrieval method, a similar image is found by using information on the distribution, kinds, or locations of colors of pixels in an image among a variety of information in the image. A leading retrieval method in color-based retrieval is an image retrieval method using MPEG-7 color descriptors which are the current standards adopted by the ISO/IEC 15938-3. The color descriptors are broadly broken down into four types: a dominant color descriptor, a color structure descriptor, a color layout descriptor, and a scalable color descriptor.
The dominant color descriptor is used when a predetermined color or a small number of colors represent the feature in the entire image or in a part of the image.
The color structure descriptor uses local color structure information together with a histogram. When characteristic information is extracted, color information on each pixel is not extracted independently, but in consideration of color information on neighboring pixels.
The color layout descriptor indicates the spatial distribution of a color. Assuming that a unit obtained by dividing an x-y plane uniformly by a predetermined size is a bin, the spatial distribution of colors existing in an image can be expressed by the number of bins with a 64-bit value. Since similarity calculation can be performed in a very simple way when the color layout descriptor is used, retrieval can be performed quickly. Also, a natural image or a color sketch image can be used as a query. Accordingly, this color layout descriptor can be appropriately applied in video browsing and retrieval.
The scalable color descriptor shows a qualitative expression of a color histogram encoded by a Harr transform, and uses a hue saturation value (HSV) color space. Since the similarity between two images is determined by the hamming distance between scalable color descriptors, similarity calculation can be performed quickly.
However, the image retrieval method using color information expressed by using the MPEG-7 color descriptors described above is sensitive to illumination. Consequently, images having identical contents may have different color information due to small differences of surrounding illuminations when the images are photographed such that image retrieval is not accurately performed. For example, a natural image may not be accurately retrieved due to a color information change caused by time and weather changes (for example, brightness and shadows in an image), a color information change caused by different illuminations (for example, an incandescent lamp or a fluorescent lamp), or a color information change occurring when an identical image is captured by different image capture apparatuses (for example, cameras produced by different manufacturers).